A nonlinear device, such as a semiconductor diode and an FET, is used as a conventional frequency conversion device. The purpose of the frequency conversion is to input a signal having a certain frequency into a frequency conversion device, and to output a signal having a frequency component different from that of the input signal from the frequency conversion device.
As a simplest example in which frequency conversion is performed by a nonlinear device, there is considered a case where current is applied to a nonlinear resistance r(i). The current-voltage characteristic of the nonlinear resistance can be subjected to Taylor expansion at a point x=i−i0 around an operating point (i0, v0) as expressed by
                              v          ⁡                      (            x            )                          =                                            i              0                        +                                                            (                                                            ⅆ                      v                                                              ⅆ                      i                                                        )                                                  i                  =                                      i                    0                                                              ⁢              x                        +                                          1                2                            ⁢                                                (                                                                                    ⅆ                        2                                            ⁢                      v                                                              ⅆ                                              i                        2                                                                              )                                                  i                  =                                      i                    0                                                              ⁢                              x                2                                      +            …                    =                                    i              0                        +                                          a                1                            ⁢              x                        +                                          a                2                            ⁢                              x                2                                      +                                          a                3                            ⁢                              x                3                                      +                          …              ⁢                                                          .                                                          [                  Expression          ⁢                                          ⁢          1                ]            When current of a sinusoidal wave with a frequency ω as expressed byx=m cos ωt  [Expression 2]is made to flow into such nonlinear resistive device, a voltage expressed byv=i0+a1m cos ωt+½a2m2(1+cos 2ωt)+¼a3m3 (3 cos ωt+cos 3 ωt)+ . . .   [Expression 3]is generated across both the ends of the nonlinear resistive device. Due to the distortion in the output waveform, harmonic wave components of frequencies 2ω, 3ω, and the like, can be extracted in addition to the component of frequency ω which is proportional to the input current.
Next, there is considered a case where the signal inputted into the nonlinear device is the sum of two signals having different frequencies ω1 and ω2. When the input current is expressed byx=ma cos ωat+mb cos ωbt  [Expression 4],a voltage expressed byv=i0+a1ma cos ωat+a1mb cos ωbt+½a2ma2(1+cos 2ωat)+½a2mb2(1+cos 2ωbt)+a2mamb{cos(ωa+ωb)t+cos(ωa−ωb)t}+ . . .   [Expression 5]is generated across both the ends of the nonlinear resistive device. Thus, it is possible to extract a frequency (ω1+ω2) which is the sum of the frequencies of the input signals, and a frequency (ω1-ω2) which is the difference between the frequencies of the input signals. In particular, the device which extracts the sum of the frequencies of the input signals is referred to as an up-converter, while the device which extracts the difference between the frequencies of the input signals is referred to as a down-converter.
In this way, the frequency conversion means to generate a signal having a frequency different from that of an input signal. The frequency conversion, which extracts a frequency of twice or an integer multiple of a certain frequency of an input signal as expressed by Expression 1, is referred to as frequency multiplication. The frequency conversion according to the present invention is assumed to include the frequency multiplication.
The frequency conversion is a very important technique. For example, a frequency conversion device is used for frequency mixing in a transmitter or a receiver in the wireless communication field. Further, a combination of a microwave oscillator and a frequency multiplication device is used to generate a millimeter wave signal or a sub-millimeter wave signal, because there is no suitable oscillator which can directly generate a signal having these frequency bands.
Generally, in the nonlinear devices used for frequency conversion, there are mainly used nonlinear characteristics exhibited by semiconductor devices, such as a diode and an FET. In many cases, a Schottky diode is used as a frequency conversion device which is used in a microwave integrated circuit (MIC) formed by mounting discrete devices on a dielectric substrate. Further, as a frequency conversion device used for frequency multiplication, a reversely biased diode is used as a nonlinear capacitive device (varactor) in many cases.
There is known a monolithic microwave integrated circuit (MMIC) which is realized by collectively and integrally manufacturing an active device, a passive device, a passive active device, and the like, on a same substrate by using a semiconductor process. In the MMIC, many FETs are used in active devices, such as an amplifier and an oscillator, and hence it is difficult to incorporate a diode designed exclusively for frequency conversion into the MMIC because of restrictions for consistency in the manufacturing process, and the like. Therefore, in many cases, the nonlinearity of FET itself is used for the frequency conversion device in the MMIC. Further, in the case where a frequency conversion device is incorporated into the MMIC, there is a restriction in the circuit area from a viewpoint of the degree of integration. Therefore, it is desired that the frequency conversion device also has a small scale. The MMIC is roughly divided into a type constituted by a Si-based device, and a type constituted by a compound semiconductor device. The Si-based device and the compound semiconductor device both have merits and demerits. However, in a monolithic microwave integrated circuit (MMIC), it is difficult to mixedly mount these devices on a same substrate. This is because in the epitaxial growth, which is necessary in the film-forming process of each of the devices in many cases, a silicon substrate is used in the Si-based MMIC, and a substrate made of GaAs, or the like, is used in the compound semiconductor. The compatibility between the manufacturing processes of the Si-based device and of the compound semiconductor device is significantly low.
Further, in general, a frequency conversion device using a semiconductor does not have the frequency selectivity in the frequency conversion device itself. Therefore, in the case where the frequency conversion is desired to be performed only with respect to a certain specific frequency, it is necessary to provide a filter, and the like. In the frequency conversion device using a semiconductor, it is not possible to provide a switching function in the frequency conversion function itself.
On the other hand, a giant magneto-resistive device (GMR) and a tunnel magneto-resistive device (TMR), which exhibit the magneto-resistive effect, have been applied as a sensor and a memory device. This utilizes the characteristics that, on the basis of the fact that the resistance value of the magneto-resistive device is changed in correspondence with the relative angle between the magnetic moments of the magnetic free layer and the magnetic pinned layer in the magneto-resistive device, a change in the external magnetic field can be detected as a change in the resistance value (sensor effect), and magnetic hysteresis is obtained as the hysteresis of the resistance value (memory effect). Further, in recent years, there has been promoted the application of devices in which spin injection torque is used in addition to the magneto-resistive effect. As described in Non-Patent Document 1, the spin injection torque means magnetic torque that is generated in a local magnetic moment by the exchange of angular momentum between a conduction electron and a local electron, which exchange is caused at the time when spin polarized current is made to flow into a ferromagnetic material. For this reason, there has been promoted the application of a microwave oscillator, a microwave detection device, a microwave amplifier, and the like, which use a non-linear effect caused by spin-injection magnetization reversal that enables magnetization reversal without using an external magnetic field, and a non-linear effect caused by the precession of magnetization induced by the spin injection torque (Patent Document 2).
The microwave detection device, which is described in Non-Patent Document 3, and the operation principle of which is based on the homodyne detection method, is capable of detecting a DC voltage corresponding to an input AC current signal. The microwave detection device uses the non-linear effect that the resistance value of a magneto-resistive device is periodically changed when the magnetic moment of the magneto-resistive device is caused to precess by the spin torque induced by the AC signal applied to the magneto-resistive device. The frequency of the change in resistance value is equal to the frequency of the inputted AC signal, and the effect as expressed by Expression 1 is exhibited. Non-Patent Document 3 describes that the homodyne detection is performed by using such non-linear effect, and also describes another important technique. That is, Non-Patent Document 3 describes a technique using a spin injection FMR effect. In the case of minute AC current, the current value is very small. Thus, the induced precession of magnetization is also very small, and hence the output DC voltage is very small. However, when the frequency of the input AC current signal is in the vicinity of a ferromagnetic resonance frequency, the precession of magnetization is amplified by the resonance effect. Thereby, a larger DC voltage can be detected. The detection function realized by using such magneto-resistive device is referred to as a spin torque diode effect. In this way, the ferromagnetic resonance is also caused by the spin injection torque, and further the nonlinear effect of the magneto-resistive device is fully exhibited by using the ferromagnetic resonance. Therefore, such effects are expected to be applied in the microwave band.    Non-Patent Document 1: Slonczewski, J. C. Current-driven excitation of magnetic multilayers. J. Magn. Magn. Mater. 159, L1-L7 (1996).    Non-Patent Document 2: Tulapurkar, A. A. et al. Spin-torque diode effect in magnetic tunnel junctions. Nature 438, 339-342 (2005).    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-295908